Try verifying if it's the PSU by removing everything except for the video card and a stick of ram. do not even connect the HDD's, floppy or CD-rom's. Basically, the only thing you should have is a mobo, video card, CPu + HSF and a ram. Power up, the system should at least post. If it post try it a few more times. If the problem goes away (intermittent on/off) it's the power supply. You can continue with your troubleshooting by adding the HDD first and then reboot and then the CD-Rom, Floppy, Soundcard, modem while rebooting at the same time on each addition of components. Find out where the system start to show it's problem again. If it only shows after adding all the components it most likely the PSU not having enough juice to power the system. Also, if you were able to power up the system with all the components replaced, check the +5.0V output of the PSU using the bios or a mobo monitor. It should not be less than 4.8V If it's below this or just close to this you definitely need to raise the +5.0V output. Some PSU's have this so check yours.